So I’ve always LOVED [name]Mace[/name]/[name]Macie[/name] for a girl, but would vastly prefer to have a longer name for paper and formal uses.
However, I can’t come up with ANYTHING. Ideas? I don’t care if it’s made up.
If you don’t mind it being made up, then I think Maciana would sound lovely (and most non-name-nerds would never suspect that it’s made up!). Good luck!
Maciana is a lovely made-up suggestion. 
[name]How[/name] about:
Macea/[name]Macia[/name]
Maciela
Massine is the French equivalent of [name]Maxine[/name].
I think [name]Marissa[/name] isn’t too far a stretch.
Honestly, when I think of [name]Mace[/name], I think of pepper spray. [name]Macy[/name], I think of [name]Macy[/name]'s. Nevertheless, [name]Macy[/name] can stand on its own [name]IMO[/name], with [name]Mace[/name] as a nickname.
The first thing I thought of was [name]Thomasa[/name], [name]Thomasina[/name], [name]Thomasin[/name], [name]Thomasette[/name], etc., where [name]Mace[/name]/[name]Macy[/name] can be directly extracted. Though honestly, I’m not a huge fan of those names. I also came up with [name]Marceline[/name], [name]Marcelle[/name], [name]Marcella[/name] and [name]Marcia[/name]. Sure, [name]Marcy[/name] would be the more obvious diminutive, but since [name]Macy[/name] is pretty much its own name, you’ve got to be a little creative especially if you don’t want to make up a name (please don’t). I know a [name]Melicia[/name] (prn. meh-LEES-ee-uh) from which [name]Mace[/name]/[name]Macy[/name] could work nicely.
Spanish name [name]Mariposa[/name] (“mary-[name]POE[/name]-suh,” means “butterfly”) could give you Masey.
What about a double name or a first name and middle name combo that could lead to [name]Macey[/name]
[name]Mabel[/name] [name]Cecelia[/name] would be the best in my opinion, but pretty much any [name]May[/name]/[name]Mae[/name] or [name]Mabel[/name] and then a C name would work.
I like the suggestion of [name]Macia[/name]. It’s a rare Polish name, and it has a similar feel to the much-beloved [name]Lucia[/name], imo.